The present invention relates to a liquid crystal display device; and, more specifically, the invention relates to an in-plane field type liquid crystal display device.
In general, an in-plane field type liquid crystal display device is provided with a pixel electrode and a counter electrode spaced apart from the pixel electrode in each pixel area on a liquid-crystal-side surface of one of the transparent substrates which are opposed to each other via a liquid crystal, and the optical transmittance of the liquid crystal is controlled by means of an electric field which is selectively generated between the pixel electrode and the counter electrode.
A liquid crystal display device of an active matrix type which adopts the in-plane field mode of operation includes, on a liquid-crystal-side surface of one of its transparent substrates, scanning signal lines extended in the x direction and juxtaposed in the y direction, counter signal lines extended in the x direction and juxtaposed in the y direction, and video signal lines extended in the y direction and juxtaposed in the x direction, and each area surrounded by adjacent scanning and counter signal lines and a pair of adjacent video signal lines is formed as a pixel area. In each pixel area, there is a thin-film transistor driven by the supply of a scanning signal from the scanning signal line and a pixel electrode to which a video signal is supplied from one of the video signal lines via the thin-film transistor. In this case, since a voltage corresponding to the video signal relative to the counter electrodes is applied to the pixel electrode, a reference voltage is applied to the counter electrode via the counter signal line.
In such a liquid crystal display device, since its structure is such that the thin-film transistors are easily damaged by static electricity, it is indispensable to take measures against static electricity. As an attempt at taking measures against static electricity, there has been a proposal in which a protection element common line is formed to surround a display area formed by an assembly of pixels, and scanning signal lines and video signal lines are connected to the protection element common line via non-linear elements at the intersections of the scanning and video signal lines and the protection element common line.
For example, a technique for taking measures against static electricity in a liquid crystal display device, which differs in structure from the in-plane field mode, is described in Japanese Patent Laid-Open No. 9-90428. In accordance with this technique, as measures against static electricity, scanning signal lines and video signal lines are connected to a common line via static protection elements. Although the static protection elements are formed of high-resistance elements so that static electricity flows through the static protection elements, the scanning signal lines and the video signal lines are electrically connected via the static protection elements and the common line. For this reason, it has been found, when the liquid crystal display device is being driven by an external circuit, part of the video signals from the video signal lines flow into the static protection elements as leakage current and this leakage current flows into the counter electrodes of the respective pixels via the common line, creating problems in that the image quality is degraded and the voltage at the video signal lines is lowered.
The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a technique which is capable of avoiding the occurrence of damage due to static electricity in a liquid crystal display device of the in-plane field type, which is provided with pixel electrodes and counter electrodes on one substrate.
Another object of the present invention is to provide a liquid crystal display device which is capable of avoiding the occurrence of leakage current from scanning signal lines or video signal lines.
The above and other objects and novel features of the present invention will become apparent from the description given herein and the accompanying drawings. A representative aspect of the invention disclosed in the present application will be summarized below.
In a liquid crystal display device, pixel areas which are surrounded by scanning signal lines, video signal lines and counter signal lines are arranged on a liquid-crystal-side surface of one of the transparent substrates opposed to each other across a liquid crystal, and the respective pixel areas include thin-film transistors driven by the supply of scanning signals from the scanning signal lines, pixel electrodes to which video signals from the video signal lines are supplied via the respective thin-film transistors, and counter electrodes spaced apart from the pixel electrodes and connected to the counter signal lines. A common line is formed to surround a display area formed by an assembly of the pixel areas, and the video signal lines and the scanning signal lines are connected to the common line by non-linear elements at the intersections of the video and scanning signal lines and the common line. The common line is connected to the counter signal lines via high-resistance elements.
In the liquid crystal display device having the above-described construction, even if leakage current due to scanning or video signals flows via the non-linear elements, this leakage current can be prevented from flowing into the counter signal lines by the high-resistance elements.